Many HO scale trolleys can run on track as narrow as 6-8" radius--have you considered simply making a loop?
The Walthers December Sale Catalog (which arrived in my mail today) has the Bachmann HO automatic-reverse trolley on sale for $109.98 plus S&H. It could be assembled in the space you described, or in a longer format. There are two variations, both complete sets:
Call 1-800-4TRAINS or go on line at www.walthers.com to order.
Chuck (modeling Central Japan in September, 1964)
You can buy a set from Bachmann, go to http://www.bachmanntrains.com/ and search on Trolley - some are reversing, some aren't.
Enjoy
Paul